1. Field of the Invention
The invention relates to a desulfurization technology for a variety of flue gases containing sulfur compounds, such as exhaust gases from combustion of heavy oil, coal and the like.
2. Description of the Related Art
As a desulfurization process for various flue gases, it is known to use magnesium components such as magnesium hydroxide and light burned magnesium oxide as a desulfurizing agent.
(1) In this process, a flue gas is first brought into contact with an absorbing liquid containing the aforesaid desulfurizing agent in a desulfurization step to cause the liquid to absorb sulfur dioxide, and the resulting absorbing liquid is treated with an oxygen-containing gas to convert magnesium salts contained therein to an aqueous solution of magnesium sulfate and sulfuric acid, the aqueous solution being then neutralized with magnesium components. The aqueous solution of magnesium sulfate after the neutralization is discharged to the sea as it is. This leads to consumption of magnesium components and sometimes requests consideration of the influence on the environment.
(2) Separately, the following process is disclosed in Japanese Patent Laid-Open No. 155263/1996 as another conventional technique in which the aqueous magnesium sulfate solution is not discharged. Namely, as shown in FIG. 7, an aqueous solution of magnesium sulfate formed in an oxidation vessel 3 is introduced into a double decomposition vessel 4 where calcium hydroxide, quick lime or the like is added to the aqueous solution to react it with the magnesium sulfate, forming gypsum and magnesium hydroxide. The mixture of these compounds is separated into a fine particle slurry comprising primarily magnesium hydroxide and a coarse particle slurry comprising primarily gypsum in an wet separator 8. The former fine particle slurry thus separated is recycled to a desulfurization column 1 as a desulfurizing agent, and a small quantity of accompanying magnesium hydroxide in the latter coarse particle slurry is treated with sulfuric acid, or with sulfuric acid formed by the oxidation of a desulfurization-step-treated liquid, or with a desulfurization-step-treated liquid while blowing air therein to convert the magnesium hydroxide to magnesium sulfate. Insoluble gypsum is separated from the slurry comprising magnesium sulfate and gypsum in a settler 9, and the aqueous magnesium sulfate solution is recirculated to the double decomposition vessel 4 for treatment.
(3) Where the mixture of gypsum and magnesium hydroxide formed in the double decomposition vesel 4 is separated into the two components by the above-described process (2), it is difficult to increase the separation efficiency due to solid/solid separation. Therefore, as shown in FIG. 8, the mixture is returned to a desulfurization column 1 without separating magnesium hydroxide from gypsum, and the magnesium hydroxide contained therein is used as a desulfurizing agent to react it with sulfur dioxide and convert it to magnesium sulfite, magnesium bisulfite and magnesium sulfate. The resulting desulfurization-step-treated liquid is sent to a gypsum separator 2 to separate insoluble gypsum by filtration. Alternatively, although not shown in FIG. 8, magnesium sulfite and magnesium bisulfite contained in the desulfurization-step-treated liquid are converted to water-soluble magnesium sulfate in an oxidation vessel 3, and the oxidation-vessel-treated liquid is sent to a gypsum separator 2 to separate insoluble gypsum by filtration.
The problem of separating solid particles into fine particles and coarse particles is solved by the above-described process (3). However, since magnesium hydroxide and gypsum are returned to the desulfurization column 1 in a mixed state, it is possible that calcium ions reacts with sulfite ions to form insoluble calcium sulfite depending on the conditions in the desulfurization column. The formation of calcium sulfite obstructs the crystallinity of gypsum, so that it not only makes difficult isolation of gypsum formed but also deteriorates the quality of the gypsum thus obtained.